Since energy production gas such as biogas is produced with the purpose of energetic use, the gas quality needs to meet the technical requirements of combustion engines. Impurities (or contaminants) such as H2S are found in concentrations between 1,000 to 10,000, while combustion systems require typically concentrations lower than 200 ppm of H2S.
The most common technologies to remove H2S from biogas are processes that are downstream of the production step. Often physio-chemical processes as wet gas oxidation are used. However, these have high investment and operational costs.
Biological gas treatment processes are based on microbial digestion of contaminants in the biological gas. Established systems for this purpose are bioscrubbers, biotrickling filters and biofilters. In general, biofiltration uses naturally occurring microorganisms to biologically break down odors, solvents and other VOCs (volatile organic compounds) present in air streams such as waste air streams or gas streams such as energy production gas streams, into carbon dioxide and waste water. It is a completely natural process that does not use chemicals or produce waste. Biofiltration is a reliable and cost-effective way to eliminate odors, VOCs and H2S at manufacturing, municipal and processing facilities. The microorganisms which reside on the surface of the biofilter support media forming a biofilm use the pollutants as a food source. A cleaned air gas stream is then discharged to the environment. Biofiltration systems need to be run without strong variations in turnover of gas and contaminants as the microbial community reacts slowly on changes and has to be balanced.
The use of biofilter systems in removal of contaminants such as, amongst others, H2S from air and gas streams has already been known for a long time. In U.S. Pat. No. 4,086,167 for instance, already dating from 1978, a biofilter for treatment of waste waters and gases, comprising a bed of coniferous tree barking residue containing microorganisms, is disclosed.
Since that time, the technology of biofiltration has been developed according to VDI guideline 3477 describing today's standards. The latest developments in biofiltration technology have amongst others been summarized in the book titled “Air pollution prevention and control: bioreactors and bioenergy”, edited by Christian Kennes and Maria C. Veiga, published 2013 by John Wiley & Sons, Ltd.
Z. Shareefdeen and A. Singh (Ed.) “Biotechnology for Odor Air Pollution Control”, Springer-Verlag (2005) have in Table 8.2, page 171 given an overview and comparison of different technologies for waste gas treatment. These are three different processes where the present invention is of the biofilter type which is a “dry” process. Biotrickling filters are operated with a wet through biofilm and in a bioscrubber process the biofilm is immersed in liquid.
TABLE B2Comparison of major technologies for waste gas treatment8 Bioscrubber Technology 171CharacteristicsBiofilterBiotrickling filterBioscrubberReactor designSingle reactorSingle reactorTwo reactorsCapital and operating costLowComparatively higherComparatively higherCarrierOrganic or syntheticSyntheticNo carrierAreaLarge area requiredCompact equipmentSmaller volume of equipmentMobile phaseGasLiquidLiquidSurface areaHighLowLowProcess controlLimited process controlLimited process controlGood process controlGas flow rate100-150 m3 m−2 h−13,000-4,000 m3 m−2 h−1OperationEasy startup and operationRelatively complicatedRelatively complicatedstartup procedurestartup procedureOperational stabilityChanneling of airflowChanneling of waterHigh operational stabilitycommonis commonPressure dropMedium to highMedium to highLowTarget compound conc.<1 gm−3<0.5 gm−3<5 gm−3Suitable for compounds<1<0.1<0.01with Henry coefficientNutrientsNutrients cannot be addedAbility to add andAbility to add andcontrol nutrientscontrol nutrientsBiomassFixed biomassFixed biomassSuspended biomassClogging of packingClogging problemClogging problemNo clogging problemExcess sludgeNo such problemsDisposal of excessDisposal of excesssludge requiredsludge required
Also, several more recent patent documents describe the use of biofilter systems in the removal of contaminants out of air and gas streams, more specifically in the removal of H2S, and more specifically out of H2S contaminated energy production gas streams containing methane.
In WO 2005/037403 for instance, a biofilter media is disclosed including grains having a hydrophilic nucleus and a hydrophobic coating including microorganisms and a metallic agent that both assist in the breakdown of amongst others H2S. The biofilter media is housed in a biofilter system including elements for the irrigation and humidification of the air stream of the biofilter media by steam or spray to ensure that the biofilter media is operating at appropriate temperature and moisture levels to avoid build-up of biomass or chemical deposits. The nutrients required for microorganism viability are therewith present in the hydrophobic coating, this preferably as a blend of trace elements. The disadvantage of the system as disclosed in WO 2005/037403 for providing the nutrients required for microorganisms viability in the hydrophobic coating is that the nutrients are not renewed once the nutrients as present in the hydrophobic coating are exhausted.
In WO 2005/005605, a system for removing H2S from methane (CH4) is disclosed which uses aerobic microorganisms to remove the hydrogen sulphide from the gas stream and oxidize it back to sulphate, which will then combine with water to form sulphuric acid. The system includes providing at least one biofilter cartridge that functions to sustain microbial activity which will function to consume H2S contained in a stream of methane gas.
Since aerobic microorganisms need an electron acceptor to be able to oxidize H2S, and in a methane gas stream, no such electron acceptors are present in sufficient amounts, air has to be injected in the biofilter cartridge as described above in order to provide oxygen to be used as the electron acceptor. As air is injected, also plenty of nitrogen is introduced into the gas stream, i.e. around 78%, diluting the methane gas and disturbing the quality thereof.
Also in US 2012/0264197, a process for removing hydrogen sulphide from a raw natural gas stream such as biogas from landfills or controlled anaerobic digestion is disclosed using oxygen, commonly in the form of air, to remove the H2S out of the raw natural gas stream. In order to solve the abovementioned problem, the natural gas stream is therewith passed through a separation unit to form on the one hand, a product stream comprising a high concentration of methane and on the other hand, a low pressure tail gas containing H2S which is passed through a biofilter including bacteria that degrades the H2S to sulphur and sulphate compounds that are washed from the biofilter.
Such a process however requires more costly equipment.
Therefore, there exists the need to provide a simple and cost effective but at the same time efficient way to remove H2S from a H2S contaminated energy production gas stream containing methane using a biofilter system, furthermore maintaining the quality of the treated (H2S depleted) energy production gas stream.